An electroluminescent (EL) arrangement is characterized in that it emits light on application of an electric voltage with flow of a current.
Arrangements of this type have been known for some time under the term “light emitting diodes” (LEDs). The emission of light occurs through positive charges (holes) and negative charges (electrons) recombining with emission of light.
All the customary LEDs in industry predominantly consist of inorganic semiconductor materials. However, EL arrangements whose essential constituents are organic materials have been known for some years.
These organic EL arrangements generally contain one or more layers of organic charge-transport compounds.
The principal layer structure is as follows:    1 Support, substrate    2 Base electrode    3 Hole-injecting layer    4 Hole-transporting layer    5 Emitter layer    6 Electron-transporting layer    7 Electron-injecting layer    8 Top electrode    9 Contacts    10 Sheathing, encapsulation.
This construction represents the most general case and can be simplified by omitting individual layers so that one layer takes on a number of functions. In the simplest case, an EL arrangement consists of two electrodes between which is located an organic layer, which takes on all functions—including that of emission of light. Systems of this type are described, for example, in WO 90/13148 on the basis of poly[p-phenylene-vinylene].
In the production of large-area electroluminescent display elements, at least one of the current-carrying electrodes 2 or 8 must consist of a transparent and conductive material.
Suitable as substrate 1 are transparent supports, such as glass or plastic films (for example polyesters, such as polyethylene terephthalate or polyethylene naphthalate, polycarbonate, polyacrylate, polysulfone or polyimide film).
Suitable transparent and conductive electrode materials are:
a) metal oxides, for example indium tin oxide (ITO), tin oxide (NESA), etc.,
b) semi-transparent metal films, for example Au, Pt, Ag, Cu, etc.
Suitable emitter layers 5 are described, for example, in DE-A 196 27 071.
However, it has been found in practice that in order to increase the luminance, electron- or hole-injecting layers (3,4 and/or 6,7) have to be incorporated into the electroluminescent superstructures.
EP-A 686 662 discloses the use of specific mixtures of conductive organic polymeric conductors, such as 3,4-polyethylenedioxythiophene and polyhydroxyl compounds or lactams, as electrode 2 in electroluminescent displays. However, it has been found in practice that these electrodes do not have adequate conductivity, particularly for large-area displays. By contrast, the conductivity is sufficient for small displays (pixel size <1 mm2).
DE-A 196 27 071 discloses the use of polymeric organic conductors, for example 3,4-polyethylenedioxythiophene, as hole-injecting layers. These enable the luminosity of the electroluminescent displays to be significantly increased compared with superstructures without the use of polymeric organic interlayers. However, the service life of these displays is still not adequate for practical applications.
EP-A 991 303 discloses 3,4-polyalkylenedioxythiophenes having particle sizes of <250 nm and conductivities of the dried polymers of <2 S/cm, corresponding to a resistivity of 0.5 Ωcm. However, for use in passive matrix displays, these 3,4-polyalkylenedioxythiophenes are still too conductive. The high conductivity results in so-called cross talk between adjacent conductor tracks (see, for example, D. Braun in Synth. Metals, 92 (1998) 107-113).
Surprisingly, it has now been found that by further reducing the size of the particles, the resistance of the 3,4-polyalkylenedioxythiophenes described in EP-A 991 303 can be significantly increased without the desired hole-injecting action being lost.